1. Field of the Invention
Many types of intermetallic compounds are known for use as hydrogen storage materials. Of particular interest herein are hydrogen storage materials provided by three-component alloys of cerium, nickel and manganese having the CaCu.sub.5 hexagonal-type crystal structure and stoichiometry.
2. State of the Art
A material suitable for storage of hydrogen must satisfy many demanding criteria. In addition to large storage capacity for hydrogen, a hydride formed during hydrogen absorption by a material should have low enthalpy characteristics; that is, the quantity of heat per mole of alloy required for formation of the hydride is preferably relatively low. Also, the hydrogen storage material should absorb and desorb hydrogen quickly, preferably at a pressure near one atmosphere and near room temperature, and with a well defined flat and the material plateau pressure region should region should show a minimum of hysteresis effects during a hydrogen absorption/desorption cycle.
One family of intermetallic compounds receiving much attention for use as hydrogen storage materials is that provided by LaNi.sub.5 -type compounds characterized by hexagonal CaCu.sub.5 crystal structure and stoichiometry. In search of improved LaNi.sub.5 -type systems, alloys have been prepared which contain other elements substituted for all or a portion of the lanthanum, but with the CaCu.sub.5 stoichiometry maintained in the new alloy. For example, in Van Vancht et al., Philips Res. Repts. 25, 133 (1970), there are described alloys such as CeNi.sub.5 and the quasi-binary system La.sub.1-x Ce.sub.x Ni.sub.5.
A comprehensive study of hexagonal AB.sub.5, that is, the CaCu.sub.5 crystal system, was reported by Lundin et al., J. Less-Common Met. 56, 19-37 (1977). Data were obtained for over 30 three-component alloys, such as lanthanum-cerium-nickel and lanthanum-nickel-copper systems. None of these alloys, however was characterized in having the combined properties of relatively low enthalpy, minimal hysteresis effects, and large hydrogen storage capacity.